The State Opposition has questioned the quality and reliability of substitute buses being used during the month long closure of Adelaide Railway Station.

The station will stay shut for just over a month, forcing thousands of commuters to use the buses for at least part of their journeys to and from the city.

The State Opposition says retired fleet buses have been called back into service to meet demand.

Opposition transport spokesman Duncan McFetridge says very hot weather over the next few days could put further stress on the system.

"The buses that are coming back in are retired buses. I hope we're not getting old clunkers out there," he said.

"We're about to get a string of 40 degree days. I don't want buses that don't have air conditioning or air conditioning breaks down.

"I don't want people in wheelchairs and those with disabilities stranded on the side of the road because the buses aren't fit for disability access."

Mr McFetridge says the buses will counteract moves to reduce congestion in the city by setting aside lanes for buses.

"Adelaide already has more buses in the CBD than any other state capital during the peak period," he said.

"We are going to see buses now on King William Street and North Terrace during the peak period. Hundreds of extra buses will be in town."

Commuters on the Gawler line have reacted angrily to the closure, with one claiming it should have been carried out while separate works forced the closure of half the line last year.

"It's ridiculous... they could have done this months and months ago all in one hit instead of closing it down and opening it up again," he said.

Station upgrade

The closure was announced last year as part of plans to electrify the rail network and upgrade the station and railyards.

City-bound commuters on the Gawler, Outer Harbor and Grange lines will complete the last leg of the trip on substitute buses at North Adelaide and Woodville.

The number of trains on the Gawler line will also be slightly reduced during peak travel periods and trips will take about quarter of an hour longer.

It is the third major interruption to the line in the past three years after services stopped at Mawson Lakes while the line was re-sleepered.

Pylons were also put in before the electrification was put on hold in last year's state budget.

The Noarlunga, Tonsley and Belair lines will be completely closed during the period and substitute buses provided.

The Noarlunga line will be closed until September as the track is re-sleepered from Adelaide to Oaklands Park.

The Belair line will not reopen until the middle of the year as the $110 million Goodwood Junction is constructed.

Adelaide Metro says Belair commuters who take their bikes on to trains will have the option of a separate service, with buses to tow trailers holding up to 12 bikes at a time.

Free travel

Transport Services Chloe Fox says substitute buses and rail journeys on the Gawler, Outer Harbor, Belair and Grange lines will be free as the work is carried out.

"I don't think it's a sweetener. I think it makes common sense to say to people when we're inconveniencing you to this extent, we need to offer you some sort of recompense," she said.

"For one month, anybody travelling on the rail up to obviously Woodville or North Adelaide, anybody travelling on a substitute bus service will have access to free public transport."

Ms Fox says while it is an unpopular move, most commuters accept that the closure is necessary.

"Of course there are going to be those who are going to inconvenienced by this, there's no way around that," she said.

"The polling that we've had shows that quite a significant majority of people understand why we're doing it, it doesn't mean they like it but they understand why we're doing it.

"This is the final stretch ahead of the electrification of our network and I'm pleased to say we're looking at the beginning of the end."

The railway station will remain closed until February 3.